CHEMICAL REACTOR DEVELOPMENT CHEMICAL REACTOR DEVELOPMENT

from Laboratory Synthesis to Industrial Production

by

Dirk Thoenes

Eindhoven University 0/ Technology, The Netherlands

Springer-Science+Business Media, B.V. A C.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN 978-90-481-4446-4 ISBN 978-94-015-8382-4 (eBook) DOI 10.1007/978-94-015-8382-4

Printed on acid-free paper

Reprinted with corrections 1998

1994 Springer Science+Business Media Dordrecht and copyright holders as specified on appropriate pages within.

All Rights Reserved © 1994 Springer Science+Business Media Dordrecht and copyright holders as specified on appropriate pages within. Originally pub1ished by K1uwer Academic Pub1ishers in 1994. Softcover reprint of the hardcover 1st edition 1994 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Preface

This book is written primarily for chemists and chemical engineers who are concemed with the development of a from the laboratory bench scale, where the fIrst successful experiments are performed, to the design desk, where the fIrst commercial reactor is conceived. It is also written for those chemists and chemical engineers who are concemed with the further development of a with the objective of enhancing the performance of an existing industrial plant. And, of course, this book is written for students of chemistry and . The term development may need some explanation. Institutes where this type of work is done are often called departments for research and development. and this type of development is then further divided into process- and product-development. The word development refers to the development of knowledge. By experimental research and by calculations, knowledge about a process is not only extended, but developed in such a sense that it can be used for designing a technical installation that is to be built for commercial purposes. A similar type of development work is aimed at the improvement of the processes that take place in chemie al plants that are already in existence. In both cases, the research and development work is aimed at predicting the behaviour of a , with the object of optimizing its performance. One may well ask: What additional knowledge does one need, after one has carried out a chemical synthesis in the laboratory, to be able to predict what will happen in a technical installation where the same synthesis is to be carried out? Do molecules not react with one another in the same manner, irrespective of the scale 0/ operation. be it in a 100 ml flask in the laboratory, in a 10 m3 vessel in an industrial plant? Or, indeed, in volumes of many cubic kilometers in the the oceans or in the atmosphere, or even in celestial bodies of huge dimensions in far away galaxies? The answer is yes, if the molecules are distributed in space in the same manner, and if temperature and pressure, which also determine the rate of chemical reactions, have the same spatial profIles. But these conditions are seldom met. When larger portions of matter, containing species that may react chemically with each other, are brought together, the distances that individual molecules will have to travel before they can react, increase, so that greater local concentration differences may occur during the reaction. If heat is evolved in the , the distance between a point where heat is generated and a wall where cooling may take place is increased accordingly, resulting in larger temperature differences within the reacting mixture. In cases where the reacting mixture flows through a reactor vessel, local pressures may again be dependent on the scale of operation. Since the results of a chemical reaction are generally determined by local concentrations, and often by pressure and temperature, the outcome of a chemical reaction is as a rule dependent on the scale on which the reaction is carried out. The consequence is that a different type of equipment may be necessary to control reaction conditions on a larger scale, but even so it may not always be possible to imitate the conditions of the small scale reactor exact1y. The altered conditions will generally result in a different composition of the product mixture coming out of the larger scale reactor, and this may have several far reaching consequences. The most important are the following: - The yield of the desired product may be different, often lower, wh ich makes the process economically less attractive. v vi PREFACE

- The selectivity of the process may be different, often resulting in more of an undesired byproduct. In many cases, this is the origin of environmental pollution. - The product quality may be different, for various reasons: - if the product is a mixture of chemical compounds (such as, e.g., motor petrol) the composition may be altered. - if the product is a polymer, which also is a mixture, the molecular mass or mass distribution of the product will be influenced. - if the product is asolid, the size, shape and structure of the product may be different. These consequences may determine the eventual success of the technical application of a new chemical synthesis. This fact is not always completely understood by laboratory chemists, and it is the aim of this book to elucidate these effects, and to present the necessary background. In general, chemical reactor development has to be aimed at increasing yields and selectivities, reducing pollution and increasing product quality. It is important to note at this stage, that the chemical interaction between molecules can be studied experimentally at any scale that is sufficiently larger than the molecular dimensions. A consequence is, for example, that unexpected side reactions, that are found when the reaction is carried out on a large sc ale (in a plant, or in the environment), may be studied in detail in the laboratory under well defmed conditions. The phenomena of which the rates are essentially scale dependent are all physical in nature, and in this context they can be summarized as physical . These phenomena can be studied separately or in combination with chemical reactions. This book focusses on the interaction between chemical reactions and physical transport phenomena. This area of science is called "Chemical Reaction Engineering", a term coined by Van Krevelen at the First European Symposium on Chemical Reaction Engineering (1957). The principles of this new area of science had shortly before been presented in three pioneering books, written by Frank-Kamenetskii (1955), Smith (1956), and Damköhler (1957). In the early sixties several textbooks appeared that since have been used widely in the teaching of this new science to chemical engineering students, namely those by Levenspiel (1962), Kramers and Westerterp (1963) and Denbigh (1965). Most of these books have since been published in revised editions and are still widely used. Since then the volume of literature in this area has grown enormously, and one can say that chemical reaction engineering has become a branch of science in its own right. This is most clearly demonstrated in the bi-annual International Symposia on Chemical Reaction Engineering (ISCRE), that are held alternately in Europe and in North America, and similar symposia held in Asia, and by the growing number of textbooks in this field. But as often happens, when a branch of science grows and diversifies, its practitioners become more specialized, and they grow apart in their interests, not only from each other, but also from the users of their branch of science. It seems almost unavoidable that fast growing branches of science gradually become less accessible to other scientists and particularly to engineers. This book is an attempt to bridge the gap that has grown between the chemie al reaction engineering science, and the area of applied chemistry and chemical process development where this science can be implemented. PREFACE vii

This book can be read in different ways. For those who are not familiar with chemical reaction engineering, it may be advisable to read flrst the elaborate Introduction presented in Part I (Chapters 1 and 2). In these chapters the scene is set and various important aspects of chemical reactions are reviewed in a qualitative sense, with their consequences for reactor development. This part contains several examples that are meant to illustrate what is going to be treated in Part IL In the end of Chapter 2 the organization of this book is presented. Part 11, General Principles, is written for those who are interested in the application of chemical reaction engineering, and wish 10 get acquainted with the most important principles. The chapters in Part 11 are organized according to physical and chemical principles. Chapter 3 is a necessary introduction about ideal reactor models, it deals with the relation between conversion and time, both in batch and in continuous reactors. Chapters 4, 5 and 6 are about physical phenomena acting on the volume element scale in chemical reactors, and their influence on the rates of chemical reactions. Studying these phenomena in small scale experiments can give important information for reactor scale-up. Chapters 7, 8 and 9 deal with integral reactor models and selection of reactor types. In Part III some Applications of chemical reaction engineering are presented. Chapters 10 - 14 are arranged according to areas of application. Several examples are given in some detail. The more experienced reader may start here, and occasionally look back at the General Principles of Part 11. This book is written with the following question in mind: "I haw a chemical reaction; how do 1 fmd the best chemical reactor?". Therefore the contents of this book are not arranged according to weIl known reactor types. The reader is not suppopsed to have chosen a reactor type apriori. Rather it is assumed that by evaluation of the available experimental data and the external requirements, the reader may be able to make a rational choice, or even develop a novel reactor type that is especially suitable for carrying out a particular reaction. By reading Chapters 3 - 9 the reader may encounter aspects relevant to reactor type swelection. When one is interested in a speciflc way for contacting reactants, one may have to select the relevant chapters and sections. For example, if the reader is interested in gas in liquid dispersions, sections 4.6.1, 5.3, 5.4, 7.1.3, 8.3.3., 10.3.1 and 12.3 may have to be consulted. The list of Contents at the beginning and the Subject Index at the end of the book may help the reader here. This book contains a number of examples, that serve to illustrate the presented theories. The examples are of a realistic nature, though they are simplifled, and presented in a generalized form: they are usually about the chemical reaction A + B -7 P + Q. The author has avoided the complications inherent to speciflc chemical examples, and hopes that the reader may recognize familiar situations. The reader should not be discouraged by the large number of mathematical equations. The mathematics does not go beyond the level that is standard in any university chemistry curriculum. The equations serve but one purpose: to be used to calculate the effects of known phenomena under altered conditions, e.g., for different concentrations, for larger scales, etc. These mathematical models serve as a tool for chemical reactor development. They may be used for the preliminary design of a reactor that is "frrst of a kind". This book is not primarily a scientific treatise, nor is it an engineering manual. It is intended as a user's handbook of chemical reaction engineering, to be utilized by those who are engaged in chemical research and development. Acknowledgements

In the wntmg of this book many persons have contributed, often without being aware of it. I am particularly indebted 10 my friend and colleague professor Wim Herman de Groot, who persuaded me 10 write this book in the first place. A special word of thanks is due 10 my collaborators at the Eindhoven University of Technology, Jan Meuldijk, Johan G.Wijers, Arend Eshuis and Huub W. Piepers, and to my graduate students, past and present, particularly Edo Ravoo, Wicher T. Koetsier, and Dirk A. Swenne, Pierre L. Geuzens, Frans J.M. Schrauwen, Gerben B.J. de Boer, Guido F.M. Hoedemakers, Coert EJ. van Lare, Karl A. Kusters, Frank J. Jeurissen, Mateo J.J. Mayer and Martien CJ. Verbruggen. Their experimental research contributed significantly to the contents of this book. Most of the ideas that lead to their research originated from numerous discussions on actual problems of chemical reactor development that I had with colleagues at DSM, Akzo and Shell. In these discussions I leamed a lot about the reality of the problems involved, for a great variety of chemical processes. In particular, the discussions I had in this context with professor Laurent L. van Dierendonck (formerly at DSM) were very instructive. I also want to thank Gerben Mooijweer (Douwe Egberts, Utrecht) for helping to solve some computational problems, and Karel Janssen for making the drawings of apparatus. I am especiallY grateful to Rita Tessmann (formerly at the University of Califomia, Los Angeles) for advising me on the correct use of the English language.

viii Contents

Preface PART I: INTRODUCTION 1. A qualitative overview of the field 1 1.1. The objectives of chemical reactor development 1 1.2. The relation between a chemical reaction and a reactor 5 1.3. The relation between the reactor and the manufacturing process 12 1.4. Conclusion 16 2. The path of chemical reactor development 17 2.1. The problem of the scale-up of chemie al reactors 17 2.2. The modelling of chemical reactors 18 2.3. Analysis of the results of laboratory experiments 20 2.4. Experiments necessary for scale-up 21 2.5. The organization of this book. 21 PART 11: GENERAL PRINCIPLES 3. Models for ideal single-phase reactors 24 3.1. Defmitions of the main concepts 24 3.2. Batch and semi-batch reactors 27 3.2.1. The ideal 27 3.2.2. The ideal semi-batch reactor 32 3.3. Continuous flow reactors 34 3.3.1. The plug flow reactor 34 3.3.2. The perfectly mixed continuous reactor 37 3.3.3 The cascade of perfectly mixed reactors 41 3.4. Selectivities in ideal single phase reactors 42 3.4.1. The concept of selectivity 42 3.4.2. The selectivity of competitive reactions 44 3.4.3. The selectivity of consecutive reactions 47 3.4.4. The selectivity of competitive-consecutive reactions 49 3.5. Homogeneous reversible reactions 50 3.5.1. Reversible reactions in batch or plug flow reactors 51 3.5.2. Reversible reactions in continuous perfectly mixed reactors 52 3.6. Conclusions on ideal single-phase reactor models 54 4. The physical contacting of reactants 56 4.1. How do moleeules get together? 56 4.2. Mixing in single-phase systems 57 4.2.1. Some general aspects of mixing 57 4.2.2. Mixing in turbulent flow 61 4.2.2.1. Some technical aspects 61 4.2.2.2. Macro-mixing 64 4.2.2.3. Turbulent micro-mixing 65 4.2.2.4. Turbulent meso-mixing 68 4.2.25. Conclusions and recommendations for turbulent mixing 71 4.2.3. Mixing in laminar flow 71 4.2.3.1. Some technical aspects 71 4.2.3.2. Laminar micro- or meso-mixing 73 4.2.3.3. Conclusions and recommendations for laminar mixing 79

LX x CONTENTS

4.3. Reactor configurations for multi-phase systems 80 4.4. Principles of between two phases 83 4.5. Mass transfer in solid/fluid systems 87 4.5.1. SolidIfluid dispersions 87 4.5.1.1. Fixed or paeked beds 87 4.5.1.2. Small particles moving through fluids by gravity 89 4.5.1.3. Solid particles suspended in stirred liquids 91 4.5.1.4. F luidized and entrained beds 93 4.5.1.5. Continuous eo- and eountereurrent solidljluid eontaeting 96 4.5.2. Continuous solid and fluid phases 98 4.6. Mass transfer in fluid/fluid systems 98 4.6.1. Dispersions of gases in liquids 98 4.6.1.1. Some general aspeets of bubble behaviour 98 4.6.1.2. Bubble eolumns 99 4.6.1.3. Stirred gaslliquid eontaetors 106 4.6.1.4. Loop reaetors 108 4.6.1.5. Gas/liquid eyclones and eentrifuges 109 4.6.2. Dispersions of liquids in gases 110 4.6.2.1. Spray towers 110 4.6.2.2. Spray eyclones 112 4.6.3. Gas/liquid contacting in parallel flow 112 4.6.3.1. Wetted wall eolumns. or falling film reaetors 112 4.6.3.2. Paeked eolumns 114 4.6.4. Dispersions of liquids in liquids 114 4.6.4.1. Teehnieal aspeets 114 4.6.4.2. Stirred liquidIliquid eontaetors 115 4.6.4.3. Agitated eolumns 115 4.7. Mass transfer in three phase systems 116 4.7.1. Possible three phase systems 116 4.7.2. Solid/liquid/gas systems 117 4.7.2.1. The most important eonfigurations 117 4.7.2.2. Slurry reaetors 118 4.7.2.3. Three phase paeked bed reaetors 120 4.8. Conclusions on the physical contacting of reactants 122 5. The interaction of chemical reactions and physical transport phenomena123 5.1. Introduction 123 5.2. Mixing and reaction in single-phase systems 126 5.2.1. Mixing and reaction times 126 5.2.2. Meso-mixing and reaction in turbulent flow 128 5.2.3. Micro- or meso-mixing and reaction in laminar flow 131 5.3. Mass transfer and chemical reaction in series 138 5.3.1. Separation of mass transfer from chemical reaction 138 5.3.2. Mass transfer and reaction at asolid surface 138 5.3.2.1. A eatalytie reaetion at a massive solid surfaee 138 5.3.2.2. The ehemieal eonversion of a massive solid 139 5.3.2.3. Mutually linked heat and mass transfer 142 5.3.3. Mass transfer in one phase and reaction in the other phase 143 5.3.4. Mass transfer followed by reaction in a large bulk 143 5.3.4.1. The steady state 143 5.3.4.2. Non-steady state ehemieal dissolution 144 CONTENTS xi

5.3.5. The influence of mass transfer on selectivity 146 5.3.6. in gas/liquid systems 146 5.4. Simultaneous diffusion and reaction in two phase systems 147 5.4.1. The general problem 147 5.4.2. Mass transfer accompanied by rapid reaction 150 5.4.2.1. Chemically enhanced absorption or extraction 150 5.4.2.2. Selectivities in chemically enhanced absorption or extraction 155 5.4.3. Diffusion and reaction in porous solids 157 5.4.3 .1. Effectiveness /actors 0/ porous catalysts 157 5.4.3.2. Selectivities in porous catalysts 160 5.4.3.3. Chemical conversion 0/ porous solids 160 5.4.4. Diffusion and reaction in fluidized beds 163 5.5. Mass transfer and chemical reaction in three-phase systems 164 5.5.1. Tbe general problem 164 5.5.2. Mass transfer at two interfaces without interaction, accompanied by chemical reaction 164 5.5.3. Mass transfer and reaction at two interfaces, with interaction 167 5.6. Conclusions on the interactions of chemical reactions and physical transport phenomena 168 6. The formation of another phase in the reactor 170 6.1. Introduction 170 6.2. Tbe effect of phase formation on process rates 171 6.2.1. Reversible reactions with phase formation 171 6.2.2. Influence of phase formation on mass transfer 172 6.2.2.1. The effects 0/ gas bubbling /rom a solution 172 6.2.2.2. The effects 0/ the precipitation 0/ solids 173 6.2.3. Tbe formation of another reaction phase 173 6.3. Tbe formation of solid reaction products 174 6.3.1. Abrief overview 174 6.3.2. Tbe precipitation of asolid product from a liquid 175 6.3.2 .1. The influence 0/ mixing on nucleation and crystal growth 176 6.3.2.2. Aggregation 178 6.3.2.3. Agglomeration: simultaneous aggregation and sur/ace ~~ 1~ 6.3.2.4. Precipitation without primary nucleation 183 6.3.3. Tbe formation of asolid product from agas 184 6.3.4. Tbe conversion of asolid reactant into asolid reaction product 185 6.3.4.1. Solid/solid reactions 185 6.3.4.2. Solid/gas/solid reactions 187 6.3.4.3. Solid/liquid/solid reactions 191 6.4. Conclusions on the formation of another phase 192 7. Integral isothermal reactor models 193 7.1. Batch and semi-batch reactors 193 7.1.1. True batch reactors 193 7.1.2. Single-phase semi-batch reactors 193 7.1.3. Two-phase semi-batch reactors 194 xii CONTENTS

7.2. Continuous reactors with one process stream 197 7.2.1. distribution in continuous reactors 197 7.2.1.1. The concepts 0/ residence time distribution and backmixing 197 7.2.1.2. Segregation versus micromixing 199 7.2 .1.3. Backmixing: residence time distribution and micro-mixing 201 7.2.1.4. The influence 0/ backmixing on selectivity 203 7.2.2. Reactors with mainly axial flow 203 7.2.2.1. Reactors with axial flow and axial mixing 203 7.2.2.2. Tubular reactors 207 7.2.2.3. reactors 207 7.2.2.4. Fluidized bed reactors 208 7.2.3. Reactors with predominant overall circulation 210 7.2.3.1. Macro mixing in stirred tank reactors 210 7.2.3.2. Complex "black box" models 211 7.3. Continuous reactors with two process streams 213 7.3.1. Various ways of contacting two process streams 213 7.3.2. Reactors with two well mixed phases 214 7.3.3. Reactors with one well mixed phase and one in unidirectional flow 216 7.3.4. Reactors with two process streams in cocurrent flow 217 7.3.5. Reactors with two process streams in countercurrent flow 217 7.3.6. Reactors with two process streams in cross flow 219 7.4. Conclusions on integral isothermal reactor models 220 8. Enthalpy management and temperature control 221 8.1. Introduction 221 8.2. Interphase heat transfer 223 8.3. Temperature control in an isothermal continuous stirred tank reactor 226 8.3.1. The adiabatic CSTR 226 8.3.2. The cooled isothermal CSTR 228 8.3.3. The isothermal CSTR with an evaporating solvent 230 8.4. Temperature control in reactors with gradients 231 8.4.1. Isothermal reactors that are not well mixed 231 8.4.2. Tubular reactors with axial temperature gradients 231 8.4.3. Tubular reactors with both axial and radial temperature gradients 232 8.4.4. Temperature control in semi-batch reactors 235 8.5. Endotherrnic processes 236 8.6. Conclusions on enthalpy management and temperature control 237 9. The selection of a reactor type 238 9.1. Introduction 238 9.2. The choice of the phase or phases that are present in the reactor 239 9.3. Selection of a reactor configuration 241 9.4. Selection of the operation mode 244 9.5. The influence of enthalpy management and temperature control 245 9.6. Conclusions on reactor type selection 246 CONTENTS xiii

PART III: APPLICATIONS 10. Reactors for organic chemical syntheses 247 10.1. Introduction 247 10.2. Single phase processes 249 10.2.1. Syntheses with complete conversion: the semi-batch reactor 249 10.2.2. Sytheses aimed at incomplete conversion 253 10.3. Two-phase organic processes 254 10.3.1. Gas/liquid processes 254 10.3.2. Liquid/liquid processes 259 10.3.3. Solid/liquid processes 259 10.4. Conclusion 261 11. Reactors for conversion or formation of inorganic solids 262 11.1. Introduction 262 11.2. Low-temperature solid/liquid processes 262 11.2.1. Chemical dissolution of solid reactants 262 11.2.2. Precipitation reactors 266 11.2.3. Reactors for simultaneous dissolution and precipitation 268 11.2.4. Electrodeposition 269 11.3. High-temperature solid/gas and solid/solid processes 270 11.3.1. Various types of processes 270 11.3.2. Conversion of asolid with agas into asolid product 271 11.3.3. The precipitation of asolid from agas 273 11.4. Conclusion 274 12. Reactors for heterogeneous 275 12.1. The design of solid catalysts in relation to reactor types 275 12.2. Reactors for catalytic gas phase processes 278 12.3. Gas/liquid/solid processes 282 12.4. Conclusion 285 13. Polymerization reactors 286 13.1. Essentials of polymerization 286 13.1.1. Polyaddition 286 13.1.2. Polycondensation 288 13.1.3. Copolymerization 289 13.2. Process configurations related to polymer recovery 290 13.3. Reactors for homogeneous polymerizations 291 13.3.1. The influence of micro-mixing on polymerization 291 13.3.2. The influence of residence time distribution and backmixing 293 13.3.3. Copolymerization reactors 295 13.4. Reactors for precipitation polymerization 298 13.5. Reactors for suspension polymerization 299 13.6. Reactors for emulsion polymerization 300 13.7. Reactors for polycondensation 303 13.8. Conclusion 306 14. Chemical reactors, product quality and the environment 307 14.1. The quality of chemical operations 307 14.2. A review of factors influencing selectivity 308 14.3. Product quality 309 14.4. The quality of waste products 313 14.5. Conclusion 315 xiv CONTENTS

Epilogue 316 Appendices to sections 318 3.3.1. The plug flow reactor: reaction mixture with varying density 318 4.5.1. Mass transfer in fluidized beds 319 5.2.3. Models for micro-mixing and reaction in laminar flow 322 5.4.2. Gas absorption accompanied by chemical reaction 326 7.3.5. Reactors with two process streams in countercurrent flow 329 List of symbols, abbreviations and units 333 Literature references and author index 339 Subject index 345

LIST OF FIGURES Figure nr. Page nr. Figure nr. Page nr. Figure nr. Page nr. 1.1 14 5.1 129 11.1 264 3.1 28 5.2 132 11.2 269 3.2 29 5.3 136 12.1 279 3.3 31 5.4 137 12.2 279 3.4 34 5.5 139 13.1 294 3.5 38 5.6 141 13.2 294 3.6 38 5.7 143 13.3 304 3.7 40 5.8 149 3A.1 319 3.8 41 5.9 149 3A.2 319 3.9 46 5.10 150 4A.1 321 3.10 47 5.11 152 4A.2 321 3.11 49 5.12 154 5A.1 325 3.12 50 5.13 156 5A.2 325 3.13 51 5.14 159 5A.3 327 3.14 51 5.15 162 7A.1 330 3.15 53 5.16 162 7A.2 331 3.16 53 5.17 166 4.1 61 7.1 198 4.2 62 7.2 199 4.3 63 7.3 200 4.4 66 7.4 202 4.5 67 7.5 206 4.6 72 7.6 210 4.7 73 7.7 212 4.8 88 7.8 214 4.9 97 7.9 218 4.10 100 8.1 227 4.11 103 8.2 227 4.12 109 8.3 232 4.13 111 8.4 237 4.14 111 4.15 119